Place the granulated sugar, butter, and brown sugar into the bowl of a stand mixer fitted with a paddle attachment. Beat on high for 2 minutes to ensure that it is creamy and fluffy. Add the eggs and egg yolks. Beat together for 30 seconds. Add the vanilla and mix.
1 ⅔ cups granulated sugar, 1 ½ cups unsalted butter, ½ cup brown sugar, 3 eggs, 2 egg yolks, 1 ½ teaspoons pure vanilla extract
Add the flour then baking powder, baking soda, and salt. Mix together until incorporated. Mix in the cocoa powder until well combined.
3 ½ cups all-purpose flour, 1 teaspoon baking powder, 1 teaspoon baking soda, 1 teaspoon salt, 1 cup cocoa powder
Shape the dough into a rough ball, wrap in plastic wrap, and chill in the refrigerator for at least 30 minutes.
Once cookie dough has chilled, preheat the oven to 350°F. Line baking sheets with parchment paper.
Remove the dough from the refrigerator and unwrap. Pinch off 2 tablespoon sized pieces and roll into balls. Place the balls onto prepared baking sheets, leaving at least 3 inches of space between each. Slightly flatten each ball into discs about 1 ½ inches in diameter.
Bake for 10-11 minutes. The centers will still be soft.
Remove from the oven and place onto a cooling rack to cool completely.
Once cooled, dip each cookie halfway into the melted white chocolate. Sprinkle immediately with peppermint pieces.
2 cups white chocolate, 1 cup peppermint pieces
Let the cookies set at room temperature for 20 minutes or in the freezer for 10 minutes to harden before serving.